This invention relates to a novel method for producing a mosaic luminescent viewing screen for a CRT (cathode-ray tube), which CRT is particularly useful for color television and color data displays.
In preparing a mosaic luminescent viewing screen by the slurry-direct photographic process, as described, for example, in U.S. Pat. No. 3,406,068 to H. B. Law, a glass support, such as the inner surface of the faceplate panel of a CRT, is coated with an aqueous slurry comprising a photoinsolubilizable binder (photobinder), a photosensitizer therefor, and particles of phosphor material. The coating is dried and then exposed to a pattern of actinic light, as by exposure through an apertured mask, to produce regions of greater and regions of lesser solubility in the coating. The coating is then developed by removing only the coating regions of greater solubility by spraying and/or flushing the coating with water or aqueous solutions under pressure, thereby producing mosaic parts of the luminescent screen. Ordinarily, three different phosphor coatings are applied, so as to produce a cyclic array of mosaic parts of three different emission colors.
The adherence of the retained less-soluble regions of each phosphor coating to the glass surface is important, and that adherence is particularly critical during the development step. The loss of even a small part of the less-soluble regions, which should constitute mosaic parts of the mosaic screen, requires the screen to be scrapped.
Under ordinary circumstances, the adherence of a phosphor coating to a clean glass surface can be improved by applying to the glass surface a very thin precoating of a water-soluble polymeric material, such as polyvinyl alcohol, prior to applying the coating. A greater adherence of the phosphor coating to a clean glass surface can be achieved by employing a heavy precoating. In one form, the heavier precoating is composed of a photoinsolubilized dichromate-sensitized polyvinyl alcohol or other water-based photosensitive organic colloid with or without phosphor particles present. In another form, disclosed in my U.S. Pat. No. 3,966,474 issued June 29, 1976, the heavier precoating is a dried layer of an aqueous emulsion of water-insoluble polymeric material, such as polystyrene particles.
After the mosaic screen is produced, it is overcoated with a specular plastic film and then with a reflecting metal layer. When either form of heavier precoating is used, relatively large amounts of organic matter of the precoating must be gasified, and the gases formed must be removed through the phosphor mosaic and the metal layer during a subsequent baking step. Also, the precoating material, which incidentally or accidentally deposits on the inner sidewalls of the faceplate panel and on the mask-mounting studs, must be removed. Excessive amounts of the gases formed and/or too rapid removal of such gases can cause blistering of the reflecting layer, which is undesirable. Also, excessive gas formation can be chemically reducing, whereby elemental carbon forms along with the gases. The novel method permits the use of a thicker precoating but reduces the volume of precoating material that is used and must later be gasified.